1. Field of the Invention
This invention relates generally to filter circuits, and more particularly relates to a filter circuit based on a novel concept which can eliminate noise only without deteriorating an original signal or derive only the desired signal from a plurality of mixed or composed signals without distortion.
2. Description of the Prior Art
To derive an original signal from a signal containing noises N.sub.1 to N.sub.3 as, for example, shown in FIG. 1A, it has been previously proposed to use a low pass filter so as to eliminate such noises N.sub.1 to N.sub.3. But, in accordance with such previous method, the original signal thus derived therefrom is blunted at its rising-up edges as shown in FIG. 1B and hence the signal is deteriorated.
Also, for a filter for separating a luminance signal Y and a chrominance signal C from a color video signal of national television systems committee (NTSC) system, there is proposed a so-called comb filter which takes advantage of the fact that a vertical correlativity of a picture screen and a phase of a subcarrier of the chrominance signal C are inverted at every one horizontal period. FIG. 2 shows a circuitry thereof, in which a composite video signal Y+C applied to an input terminal 101 is supplied to an adding circuit 102 and a subtracting circuit 103, and the signal applied to the input terminal 101 is supplied through a one horizontal period (1H) delay circuit 104 to the adding circuit 102 and the subtracting circuit 103. Then, the signals 2Y and 2C derived from the above adding circuit 102 and the subtracting circuit 103 are respectively delivered through attenuators 105 and 106, each used to attenuate the level of the signal applied thereto to be one-half the original signal, to output terminals 107 and 108.
Next, the operation of the prior art comb filter shown in FIG. 2 will be described. Now, let us assume that, as shown in FIG. 3A, the input terminal 101 is supplied with such a signal including, for example, five successive scanning lines components, in which first and second signals i and j thereof are only the luminance signal Y of a constant level, while third, fourth and fifth signals k, l and m thereof are a mixed or composed signal of the luminance signal Y and the chrominance signal C, both being at the constant level. In the above, it is also assumed that a signal preceding the first scanning line be equal to the first scanning line signal i.
When the input terminal 101 is supplied with the signal denoted by letter i in FIG. 3A, at the output of the delay circuit 104, there appears a signal with a waveform same as that of the signal i one horizontal period (1H) before. Accordingly, the output signal from the adding circuit 102 becomes a signal with an amplitude twice that of the signal i. This signal is supplied to and attenuated to be one-half in level by the attenuator 105, so that at the output terminal 107 is developed such a signal as shown by letter i' of FIG. 3B, which is the same as the first scanning line signal i of FIG. 3A. Moreover, the output of the subtracting circuit 103 becomes zero so that at the output terminal 108 is produced a signal whose luminance signal component is removed as shown by letter i" in FIG. 3C. Next, when the input terminal 101 is supplied with the signal denoted by letter j of FIG. 3A, similarly to the above, at the output terminal 107 is produced the luminance signal j' shown in FIG. 3B, while at the output terminal 108 is produced the signal j" of FIG. 3C containing no luminance signal Y.
When the input terminal 101 is supplied with the signal denoted by k in FIG. 3A, the 1H delay circuit 104 produces the signal shown by j in FIG. 3A. Accordingly, at the output from the adding circuit 102, the level of the luminance signal Y becomes twice high the original level, while the level of the chrominance signal C is the same as that of the signal k. Therefore, at the output terminal 107 is produced a signal k' shown in FIG. 3B where the level of the luminance signal Y thereof is the same as that of the input signal k in FIG. 3A, and the level of the chrominance signal C is one-half that of the input signal k. Furthermore, from the output terminal 108 is derived a signal shown by k" in FIG. 3C where the chrominance level is one-half and the luminance signal component is eliminated.
Next, when the input terminal 101 is supplied with the signal denoted by letter l in FIG. 3A, the 1H delay circuit 104 produces the signal k. Comparing the signal k with the signal l, it is seen that the luminance signals Y thereof are in the same phase and in the same level, while the chrominance signals C thereof are in the opposite phase, but the same level, so at the output terminal 107 is produced only the luminance signal Y with the same level as that of the input signal as shown by letter l' in FIG. 3B. Whereas, at the output terminal 108 is produced only the chrominance signal C with the same level as that of the input signal as shown by letter l" in FIG. 3C.
Next, when the input terminal 101 is supplied with the signal denoted by letter m in FIG. 3A, similarly to the signal l, at the output terminal 107 is produced only the luminance signal Y as shown by m' in FIG. 3B, while at the output terminal 108 is produced only the chrominance signal C shown by m" in FIG. 3C.
Specifically, in the prior art comb filter shown in FIG. 2, when the signals in the adjacent scanning lines have a vertical correlativity therebetween, it is possible to perfectly separate the luminance signal Y from the chrominance signal C. But, when they have no vertical correlativity therebetween or, for example, when the signal j of the second scanning line is changed into the signal k of the third scanning line, as shown by the signal k' of FIG. 3B, the chrominance signal C is mixed into the output terminal 107 for providing the luminance signal Y, to cause a dot interference. Also, as shown by the signal k" of FIG. 3C, the level of the chrominance signal C is attenuated to be one-half the original level thereof thus the vertical resolution being deteriorated.